Electrical barrier structure for semiconductor device doped with chromium and/or titanium

ABSTRACT

A semiconductor device, preferably a laser device such as a signal generator, a signal amplifier or a signal detector e.g. a distributed feedback laser, which is implemented in III/V semiconductors. Such devices often require a barrier layer to encourage current flow to pass through the localised p/n-interface and this invention provides the barrier layer in the form of a layer of hole trapping semiconductor material located between and in contact with two p-type layers. III/V semiconductors contain at least one of indium, gallium and aluminum and at least one of phosphorus and arsenic but the preferred devices are laser devices implemented in various types of indium phosphide except for the active zone wherein photons are generated. The active zone is preferably formed of ternary and/or quaternary semiconductors. In the preferred structures the barrier layer is formed of chromium doped indium phosphide which is located between two layers of p-type indium phosphide. Alternative structures have (a) titanium doped indium phosphide between two layers of p-type indium phosphide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor devices and especially to laserstructures which structures include barrier regions to regulate the flowof electric current in order to enhance the performance of the device.

The invention includes purely electronic devices, which utilise abarrier layer in order to preferentially channel current through anactive region. However, the preferred devices are laser devices in whichthe barrier layers modify the flow of current in order to improve theefficiency, eg. the efficiency of conversion of electric power intoradiated power. The barrier has a secondary effect in that the laserdevice is able to operate at higher rates of modulation.

The laser devices mentioned above can be utilised to carry out a varietyof different telecommunications and signalling functions.

For example, the laser device can be utilised to generate opticalsignals. For this application, the laser device needs to be associatedwith a reflective or resonating structure which returns photons into theactive region in order to sustain the lasing process. Resonating systemswhich are highly wavelength selective are particularly appropriate sincethey narrow the bandwidth of the optical signals which are generated.External resonating systems may be utilised or the resonating system maybe incorporated into the lasing structure, eg. as in a distributedfeedback laser. Cleaved facets, as in Fabry-Perot lasers may also beused for feedback.

The laser device may also be utilised for other signalling andtelecommunication functions, e.g. optical amplifiers and opticaldetectors. In devices of this nature reflections are undesirable sincethey tend to produce noise. Therefore, it is appropriate to provide thelasing device with anti-reflection coatings in order to reduce, andideally to eliminate, reflections.

2. Description of Related Art

From the above description, it will be recognised that there are a widevariety of electronic and opto-electronic semiconductor devices whichrequire barrier layers to control the flow of current, (e.g. asdescribed in 3rd International Conference on Indium Phosphide & RelatedMaterials, Jowett et al "The Growth of MQW Planar Buried HeterostructureLasers with Semi-Insulating Blocking Layers by OMVPE"; April 1991 pp208-211). In particular, it is necessary to provide barrier layers insemiconductor devices wherein the semiconductors contain at least one ofindium, gallium and aluminium and at least one of phosphorous andarsenic. In addition to the five major elements just mentioned thesemiconductors also contain small quantities of dopants in order toconfer particular electrical properties, e.g. p-type conduction orn-type conduction. Semiconductor structures wherein the various layerscontain at least one of the elements indium, gallium and aluminium andat least one of the elements arsenic and phosphorus (together withdopants needed to provide the appropriate electrical function) areconveniently called III/V-devices.

The device structures usually include localised p/n-junctions wheren-type regions contact p-type regions and, in order to encourage flowthrough a localised junction it is appropriate to provide an electricalbarrier elsewhere. This invention relates to the electrical barrier.

SUMMARY OF THE INVENTION

It has been recognised that III/V semiconductor materials can be dopedto trap either holes or electrons. For example, indium phosphide dopedwith chromium and/or titanium traps holes whereas indium phosphide dopedwith iron traps electrons. In such doped semiconductors that part of theconduction which depends on the trapped species is low but theirperformance as a barrier is disappointing, e.g. they permit more currentflow than is desirable.

This invention is based on the discovery that the barrier properties oftrapping III/V semiconductor regions are substantially improved whensaid region is located between and in contact with two III/Vsemiconductor regions each of the opposite type to the centre region. Itwill be appreciated that a barrier is usually required between p-typeand n-type regions and in such a location the barrier is inherently incontact with one p-type and one n-type layer. Thus, the structure of theinvention is not inherently provided because, on one side, the trappinglayer is in contact with material of the same type as itself. In thesecircumstances, it is necessary to provide an extra layer of p-typematerial located between said hole-trapping region and the n-type regionin order to achieve the desired configuration.

The invention, which is more fully defined in the claims, includes III/Vsemiconductor devices having an electrical barrier configured as aregion of a hole trapping semiconductor located between and in contactwith each of two p-type semiconductor regions. In many devices theelectrical barrier is located between regions implemented in variouslydoped forms of indium phosphide and, in such locations, it is convenientthat all parts of the barrier be implemented in various types of indiumphosphide.

It is now appropriate to comment on the factors affecting theconcentration of the dopant used for trapping. It is undesirable to riskjeopardising the crystal structure by the use of very highconcentrations of dopant, e.g. if the dopant is not uniformlydistributed or if the single crystal structure of the device isdisrupted. It appears that there is an upper threshold for theconcentration of the dopant. Below this upper threshold, higherconcentrations appear to increase the trapping effect but there appearsto be little, if any, extra trapping to be gained by increasing theconcentration of the dopant above its upper threshold. Nevertheless, itis often appropriate to use concentrations which exceed the upperthreshold, e.g. 1 to 50 times or even 1 to 100 times the upperthreshold, in order to ensure that a maximum effect is obtained.

It should be recognised that there is a solubility limit for the dopantin the semiconductor. At and below this limit the composition is uniformand stable, but the composition may still be metastable at evensubstantially greater concentrations, e.g. up to 100 times as mentionedabove. The preferred configuration in accordance with the inventiontakes the form of a layer of hole-trapping indium phosphide, eg chromiumor titanium doped indium phosphide, located between and in contact withtwo layers of p-type indium phosphide.

It is emphasised that both chromium and titanium are good for thetrapping of holes in indium phosphide. Chromium is more convenient thantitanium for use in metal organic vapour phase epitaxy (MOVPE)deposition. For example, bisbenzene chromium, (C₆ H₆)₂ Cr, is anorganometallic compound which is convenient for use as a reagent inMOVPE. This reagent causes no side effects or problems and the chromiumdoes not engage in any unacceptable side reactions under the conditionswhich appertain in an MOVPE reactor. As mentioned, chromium traps holesin indium phosphide sufficiently to give effective barriers inaccordance with the invention. The preferred concentration range ofchromium is 10¹⁵ -10¹⁹ atoms/cc and the solubility limit is about 2×10¹⁶atoms/cc.

Titanium is even more effective than chromium in trapping holes inindium phosphide; possibly because indium phosphide will tolerate higherconcentrations of titanium. Titanium formstetrakis(dimethylamino)titanium, Ti N(CH₃)₂ !₄, which is suitable foruse in MOVPE reactions. However, titanium is more reactive than chromiumand, even at the low level of impurities acceptable in MOVPE reactionsystems, it is possible for titanium to engage in unwanted sidereactions. These side reactions can be reduced to an acceptable levelbut this requires extremes of technique and carefulness from theoperators and, because of the reactivity of titanium, the preferreddopant is chromium.

The III/V devices in accordance with the invention are particularlysuitable for implementation as laser devices which include an activezone wherein the generation of photons actually occurs. A wide varietyof different active zones are known for different purposes. For examplethe active zone may be homogenous or it may contain several differentthin layers as in multi-quantum well structures and strainedsuper-lattice structures. (In a multi-quantum well structure thethickness of the layers is selected to produce quantum effects whereasin a strained super-lattice the layers have different crystallographicparameters in order to produce a strained lattice which affects theperformance in a desired manner. It is possible to have an active regionwhich is both multi-quantum well and strained super-lattice.) In manycases the active zone is implemented in ternary semiconductors, eg.containing indium and gallium with arsenic or phosphorus or quaternarysemiconductors, ie. containing all four of indium, gallium, phosphorusand arsenic. It will be appreciated that a multi-layered structure willcontain at least two different compositions, eg. ternary and quaternarylayers. The active zone is usually located close to a p/n-junction but,in some cases, the active zone includes the p/n-junction. In manydevices a substantial proportion of the volume, eg. except for theactive zone and for contact layers, is implemented in various forms ofdoped indium phosphide.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a transverse cross-section illustrating a buriedheterostructure which includes a barrier region in accordance with theinvention;

FIG. 2 is a longitudinal cross-section through a device as shown in FIG.1 implemented as a distributed feedback laser,

FIGS. 3a is perspective, partly cut away, view of the device illustratedin FIG. 2;

FIG. 3b is an enlargement, showing greater detail of the indicated cartof FIG. 3a: and

FIG. 4 illustrates in longitudinal cross-section, the device of FIG. 1implemented as an optical amplifier with anti-reflection coatings.

FIG. 5, which is for comparison, is a transverse cross-sectionillustrating a buried heterostructure which includes an iron dopedbarrier region.

DETAILED DESCRIPTION OF THE DRAWINGS

As shown in FIG. 1, the device comprises a substrate 10 formed of n-typeInP and, supported thereon a first region 11 which is also formed ofn-type InP. Region 11 supports an active zone 19 and p-doped layer 23.

Regions 11, 19 and 23 are shaped as a mesa which projects into a secondregion 12 which is formed of p-type InP. The mesa is two-sided and eachside has a base 13 with a curve 14 connecting to the plateau 15 of themesa.

The active zone 19 is where the generation of photons occurs. The p/njunction of the device is within or close to the active zone 19. Severalcompositional and locational possibilities exist for the active zone 19.

As regards location, the active zone 19 is on the first region 11 andone of its faces may be co-incident with the plateau 15. Alternatively,the active zone 19 may be below the plateau 15 (in which case thecovering layer 23, of p-type InP, lies between the active zone 19 andthe plateau 15). As regards composition, the active zone 19 may behomogenous or it may be formed of several layers, eg. it may be amulti-quantum well zone and/or a strained super lattice. Ternarysemiconductors (containing both of indium and gallium and one ofphosphorus and arsenic) and quaternary semiconductors (containing allfour of indium, gallium, phosphorus and arsenic) are suitable forforming the active zone 19 or the various layers thereof.

Usually the active zone has a higher refractive index than itssurroundings. Thus it acts as a confining region for the photonsproduced therein or, in the case of amplifiers and detectors, suppliedfrom outside. Usually the active zone extends from one facet to theother and it constitutes a waveguide between these two facets.

As implied above, the p-type region of the device makes contact with then-type region at a localised p/n-junction which is within or close tothe active zone 19. For efficient operation, eg. for the efficientconversion of electric power into radiation, it is desirable to directas much as possible of the electric current so that it flows through theactive zone 19. In order to encourage this, it is desired to discouragethe current flow elsewhere and a barrier region 18 is therefore locatedbetween the p-type and the n-type regions.

If the active zone 19 extends longitudinally throughout the structurethe barrier region 18 consists of two separate sub-regions, ie. one oneach side of the active zone 19. Each sub-region of the barrier 18 isformed of two layers, namely a thin layer 16 adjacent region 11 and athicker layer 17 adjacent to region 12.

In the preferred embodiment, layer 16 is formed of p-type indiumphosphide and layer 17 is formed of chromium doped indium phosphide. Thechromium doping is at a concentration of 10¹⁸ atoms per cubic centimetre(ie. 50 times the upper threshold). The effect of the chromium doping isto make the hole conduction of layer 17 very low and it is very close tozero. It should be noted that the chromium doped layer 17 is locatedbetween and in contact with two p-type regions of indium phosphide. Oneof these regions is provided as region 12 (which is part of theoperational structure) but the other, ie. layer 16, is an additionallayer. It is emphasised that it is the combination of three layers, ie.12, 16 and 17 which, in accordance with the invention provide goodbarrier properties. Without layer 16, electrons from layer 11 would beinjected through layer 17 causing substantial conduction.

By way of comparison a different blocking structure is illustrated inFIG. 5. Basically this is the same as FIG. 1, but layers 16 and 17 arereplaced by an iron doped layer 34 directly on layer 11 with a thinn-doped layer 33 directly above layer 34. Mostly this serves as a goodblocking structure, ie it has an electron trapping layer located betweentwo n-type layers. However, in the vicinity of the mesa plateau 15,there exists a region with p-type contacting directly to iron dopedmaterial. This allows substantial hole injection, and reduces theefficiency of the barrier. In the present invention shown in FIG. 1,there is no such equivalent injection path ie n-type to chromium dopedmaterial.

To complete fabrication of the device, an external contact is providedon top of second region 12. This consists of a central conductive stripe20 located above the active zone 19 The stripe 20 is usually formed ofInGaAs doped for high conductivity and it is in contact with secondlayer 12. Two silica regions 21 are located, one on each side of, thestripe 20. The silica constitutes an insulator. Finally a metallicterminal 22T covers the whole or part of the top surface. This metallicterminal makes electrical contact with the stripe 20 and it isconveniently formed of titanium and platinum and gold layers or titaniumand gold layers.

A metallic terminal 22B also of titanium and gold layers is provided onthe base of the substrate. When the device is in use, external contactsare soldered to these terminal layers 22T and 22B. It should be notedthat the operational structure, ie. layers 10-19, is located between theexternal contacts, ie. items 20-22T and item 22B. The functionalstructure is formed of various types of III/V semiconductors.

A brief description of the preparation of the structure illustrated inFIG. 1 will now be given. This utilises conventional MOVPE, conventionalmasking and conventional photolithography. The preparation starts withthe substrate 10 and, using conventional MOVPE techniques, the firstregion 11, the active zone 19 and the covering layer 23 are grown insequence. At the end of the growth period the substrate 10 was coveredby layers of uniform thickness providing the first region 11 and thevarious layers of the active zone 19 and the covering layer 23. Havinggrown layers of uniform thickness it is necessary to shape these toproduce the mesa as illustrated in FIG. 1.

As a preliminary to shaping, 200 nm of silica were deposited and, usingconventional photoresists, a protective stripe was placed above theintended plateau of the mesa. This allowed the side portions of thesilica to be removed. This exposes the semiconductor which is etched toa depth of 2 μm.

The next step is to grow the barrier layers 16 and 17. During the growthof these layers the silica remains on the plateau of the mesa to preventgrowth thereon. Layer 16, ie. 100-200 nm of zinc doped InP is grown onthe base 13 and curved sides 14 of the mesa. After the deposition oflayer 16, 2 μm of chromium doped InP was grown using bisbenzene chromiumin a conventional bubbler to supply the chromium to the MOVPE reactor.At this stage the silica stripe was removed from the mesa usinghydrofluoric acid and, after said removal, the region 12, ie. 1.5 μm ofp-doped InP and region 20 were grown.

Finally the contact regions 21 and 22 were provided by conventionaltechniques.

FIG. 2 is a longitudinal cross-section through the active zone 19 of aDFB laser incorporating the structure as shown in FIG. 1. The barrierdoes not appear in FIG. 2 because it is not present in this crosssection. However, this figure shows the location of a second ordergrating 30 providing resonance at a wavelength of 1.55 μm. It should benoted that the grating 30 can be provided in the covering layer 23.

FIGS. 3a and 3b are cutaway perspective views showing the device of FIG.2 in greater detail. Since all the parts have already been identified nofurther description is necessary.

FIG. 4 is a longitudinal cross-section through a device as shown in FIG.1 but implemented as an optical amplifier. Since reflections areundesirable for this function there is no grating or any reflective orresonance system. Instead the end faces of the device are provided withanti-reflection coatings 31 and 32 so as to minimise reflections as muchas possible. (Note. The anti-reflection coatings are conventional andtheir structure is usually multi layer. Some devices have only oneantireflection coating.)

In order to illustrate the advantage of the invention, a laser accordingso the invention was compared with a laser similar to that shown in FIG.1 but prepared using a single iron doped indium phosphide barrierinstead of barrier layers 16 and 17. In other words the barrier isconstituted by iron doped indium phosphide instead of the barrier inaccordance with the invention.

Both lasers had a threshold bias current of 15 mA; ie. at bias currentsbelow 15 mA no light output was produced. At bias currents of 15 mA verysmall light output is produced. but there is not enough output todisplay the advantage of the invention.

Table 1 contains comparative measurements for bias current in milliampsgiving: -

Column 1: bias current in mA:

Column 2: light output in milliwatts for prior art (Fe),

Column 3: light output in milliwatts for the invention (Cr), and

Column 4: the percentage improvement.

    { (col 3)-(col 2)!/(col 2)}*100.

                  TABLE 1                                                         ______________________________________                                        mA        Fe            Cr     %                                              ______________________________________                                        40        3.2           4.0    25                                             60        5.0           6.8    36                                             80        7.2           9.8    36                                             100       8.6           12.2   41                                             ______________________________________                                    

Table 1 illustrates that the barrier layer in accordance with theinvention is substantially better than the equivalent barrier of irondoped InP giving 30-40% improvements in the power of the light outputfor similar bias currents.

It is believed that a structure as shown in FIG. 5 would have aperformance similar to column 2 of Table 1.

We claim:
 1. A III/V semiconductor device which includes a p-type regionand an n-type region with an electrical barrier located between saidp-type region and said n-type region characterised in that said barriertakes the form of two layers of III/V semiconductor, namely a firstlayer doped with chromium adapted for trapping holes and a second layerof a p-type III/V semiconductor material wherein said first layer isbetween and in contact with said second layer and the p-type region ofthe device.
 2. A device according to claim 1, wherein the first layer isformed of indium phosphide doped for hole trapping and it is in contactwith two layers each of which is formed of p-type indium phosphide.
 3. Adevice according to claim 1, which comprises first and second contactzones each of which includes a metal layer constituting an externalcontact of the device and a functional structure located between saidfirst and second contact zones wherein said functional structure isformed of one or more regions each of which is formed of a semiconductormaterial which contains at least one of indium, gallium and aluminiumand at least one of phosphorus and arsenic.
 4. A semiconductor deviceaccording to claim 3, wherein the functional structure is a laserstructure which comprises:(a) a localised p/n-junction; (b) an activezone wherein photons are generated, and wherein (a) is within or closeto the active zone; (c) a region of p-type indium phosphide; (d) aregion of n-type indium phosphide; and (e) an electrical barrier whichis located between (c) and (d) so as to cause current to flow through(a) in preference to elsewhere; characterized in that:(e) comprises twolayers (e1) and (e2), (e1) is formed of hole-suppressed indium phosphidedoped with chromium; (e2) is formed of p-type indium phosphide; (c) isin contact with (e1), (e1) is also in contact with (e2), and (e2) isalso in contact with (c) whereby (e1) is located between and in contactwith two layers of p-type indium phosphide.
 5. A device according toclaim 4, wherein (e1) is formed of indium phosphide doped with chromiumat a concentration between 10¹⁵ and 10¹⁹ atoms per cubic centimetre. 6.A device according to claim 4, wherein (b) has a uniform compositionsaid composition being a doped semiconductor containing both of indiumand gallium and at least one of phosphorus and arsenic.
 7. A deviceaccording to claim 4, wherein (b) is formed of a plurality of layers ofsemiconductor materials to form either a quantum well structure and/or astrained super lattice wherein each of said layers is a semiconductorcontaining both of indium and gallium and at least one of phosphorus andarsenic.
 8. A device according to claim 4, which is a burnedheterostructure device having (b) located at or near a plateau of a massprojecting into (c).
 9. A device according to claim 4, wherein the laserstructure is a distributed feedback laser and said device includes agrating structure located for co-operation with (b).
 10. A deviceaccording to claim 4 which is a Fabry-Perot laser device having twocleaved facets as reflectors.
 11. A device according to claim 4, whichis a semiconductor amplifier having anti-reflection coatings on one orboth facets in order to suppress reflections.
 12. A device according toclaim 1 wherein the barrier comprises two separate subregions whichdefine a gap in the barrier, a localised p/n junction located in the gapwherein the p-type region of the device contacts the n-type region ofthe device, and the p-type and n-type regions are separated elsewhere bythe barrier.
 13. A method of making a device as in claim 1 comprisingforming the n-type region on a substrate;forming an active zone ofsemiconductor material on the n-type region; etching the active zone andthe n-type region to form a mesa shaped structure; forming saidelectrical barrier on the mesa shaped structure; and providing thep-type region on the barrier layer and the active zone, whereby thefirst layer of the electrical barrier is located between and in contactwith two layers of p-type semiconductor material of the device.
 14. AIII/V semiconductor device which includes a p-type region and an n-typeregion with an electrical barrier located between said p-type region andsaid n-type region characterized in that said barrier takes the form oftwo layers of III/V semiconductor, namely a first layer doped withtitanium in the absence of iron adapted for trapping holes and a secondlayer of a p-type III/V semiconductor material wherein said first layeris between and in contact with said second layer and the p-type regionof the device.
 15. A device according to claim 14 wherein the firstlayer is formed of indium phosphide doped for hole trapping and it is incontact with two layers each of which is formed of p-type indiumphosphide.
 16. A device according to claim 14 which comprises first andsecond contact zones each of which includes a metal layer constitutingan external contact of the device and a functional structure locatedbetween said first and second contact zones wherein said functionalstructure is formed of one or more regions each of which is formed of asemiconductor material which contains at least one of indium, galliumand aluminum and at least one of phosphorus and arsenic.
 17. Asemiconductor device according to claim 16 wherein the functionalstructure which comprises:(a) a localized p/n junction; (b) an active azone wherein photons are generated, and wherein (a) is within or closeto the active zone; (c) a region of p-type indium phosphide; (d) aregion of n-type indium phosphide; and (e) an electrical barrier whichis located between (c) and (d) so as to cause current to flow through(a) in preference to elsewhere; characterized in that:(e) comprises twolayers (e1) and (e2); (e1) is formed of hole-suppressed indium phosphidedoped with titanium in the absence of iron, (c) is in contact with (e1),(e1) is also in contact with (e2), and (e2) is also in contact with (c)whereby (e1) is located between and in contact with two layers of p-typeindium phosphide.
 18. A device according to claim 17 wherein (b) has auniform composition said composition being a doped semiconductorcontaining both of indium and gallium and at least one of phosphorus andarsenic.
 19. A device according to claim 17 wherein (b) is formed of aplurality of layers of semiconductor materials to form either a quantumwell structure and/or a strained super lattice wherein each of saidlayers is a semiconductor containing both of indium and gallium and atleast one of phosphorus and arsenic.
 20. A device according to claim 17which is a burned heterostructure device having (b) located at or near aplateau of a mass projecting into (c).
 21. A device according to claim17 wherein the laser structure is a distributed feedback laser and saiddevice includes a grating structure located for co-operation with (b).22. A device according to claim 17 which is a Fabry-Perot laser devicehaving two cleaved facets as reflectors.
 23. A device according to claim17 which is a semiconductor amplifier having anti-reflection coatings onone or both facets in order to suppress reflections.
 24. A deviceaccording to claim 14 wherein the barrier comprises two separatesubregions which define a gap in the barrier, a localized p/n junctionlocated in the gap wherein the p-type region of the device contacts then-type region of the device, and the p-type and n-type regions areseparated elsewhere by the barrier.
 25. A method of making a device asin claim 14 comprising forming the n-type region on a substrate;formingan active zone of semiconductor material on the n-type region; etchingthe active zone and the n-type region to form a mesa shaped structure;forming said electrical barrier on the mesa shaped structure; andproviding the p-type region on the barrier layer and the active zone,whereby the first layer of the electrical barrier is located between andin contact with two layers of p-type semiconductor material of thedevice.